As a fluid container for accommodating fluid such as liquids, viscous substances, granules and powders, there is conventionally known a piston/cylinder type fluid container comprising a tubular container main body having at least one end closed by an end wall, and a piston member internally received in the container main body in an axially slidable manner, thereby defining a fluid containing chamber having an annular cross section in cooperation with the end wall. Such fluid containers are disclosed, for instance, in Japanese patent laid open publication (Kokai) Nos. 59-37162 and 59-37163.
Such a fluid container is used, for instance, as an ink bottle for a printer such as a stencil printer, and its content or printing ink can be taken out of the container by suction via a content outlet provided in the tubular container main body connected to a suction pump.
In such a process of taking out the content by suction, the piston member axially moves inside the tubular container main body following the expulsion of the content out of the content outlet, but, as the inner diameter of the tubular container main body is increased to meet the demand for a larger capacity of the container, the piston member acquires the tendency to slant with respect to the axial line, thereby causing a sticking between the piston member and the cylinder wall. Once the piston member becomes stuck at a middle point in its axial movement inside the tubular container main body, the content inside the tubular container main body will be no longer available for use.
In view of such a problem of the prior art, the same applicant has proposed in Japanese utility model application No. 2-123755 a fluid container comprising an internal tubular member axially and securely fitted inside a container main body, and a piston member internally received in the container main body and externally fitted on the content expelling tube in an axially slidable manner, thereby defining a fluid containing chamber having an annular cross section in cooperation with an end wall so that a fluid containing chamber having an annular cross section may be defined inside the container main body and the end wall, and the axial movement of the piston member may be guided by the internal tubular member.
According to such a fluid container provided with an internal tubular member, a desired object can be achieved. Further, according to such a fluid container provided with an internal tubular member, it is possible to communicate the interior of the content expelling tube with the interior of the container main body at an end wall side of the container main body, and to provide a content outlet at an end of the content expelling tube.
However, in the case where the content expelling tube is given with the additional function as a member for defining the content outlet, after the piston member has reached its stroke end adjacent to the end wall of the container main body, the content remaining in the content expelling tube cannot be expelled, thereby wasting the part of the content finally remaining in the content expelling tube.
According to such a piston/cylinder type fluid container, the content can be taken out with less waste as compared to the more conventional fluid container such as a laminated tube, but, on the other hand, once the content of the fluid container is all taken out as designed, there is no way to take out the content therefrom any further no matter how attempts are made. On the other hand, in the case of a laminated tube, even after the content is depleted in normal sense, it is still possible to squeeze the laminated tube hard, and force out a small amount of content such as printing ink which may serve the purpose until a new laminated tube is made available.
Therefore, for instance, in the case of an ink bottle of a printer from which the content or printing ink is taken out by using power means, unless a new bottle is prepared before the content of the current ink bottle is depleted, a situation may arise in which printing becomes totally impossible without any warning.
To avoid such an inconvenience to occur, the operator must be always aware of the remaining amount of the content of the container, and this not only imposes a burden on the operator but is intrinsically unreliable.
In such a cylinder/piston type fluid container, as means for detecting the depletion or the imminence of the depletion of the fluid content, it has been proposed, for instance, in Japanese patent laid open publication (Kokai) No. 59-37163, to detect the position of the piston member by providing a sensing portion consisting for instance of a magnetic member in the piston member for detection by a non-contact type sensor for detecting the position of the piston member such as a reed switch placed at a stroke end of the piston member or a position adjacent thereto.
However, according to such a method for detecting the amount of fluid content remaining in a fluid container, it is necessary to detect the movement of the piston member in a non-contact manner and, to achieve a high precision in detection, it is necessary as a matter of course to precisely position the sensing means with respect to the fluid container. Further, when the sensing means is to be placed on one side of the fluid container for detecting the movement of the piston member, it is necessary for the piston member to move without inclining in the radial direction or without any rocking movement, and the sensing portion such as a magnetic member must be placed over the entire circumference of the piston member.
The internal cross section of the fluid container or the outer diameter of the piston member increases as the volume of the fluid content in the fluid container is increased, and the increase in the outer diameter of the piston member increases the tendency of the piston member to incline in the radial direction, and the increased length of the outer circumference of the piston member means the need for a larger amount of material for the sensing portion.
As the internal cross section of the fluid container increases, the speed of the movement of the piston member involved in taking out the content becomes lower. Such a slow movement of the piston member is advantageous when quantitatively and continually detecting the position of the piston member, but, when the arrival of the piston member at a certain position with an on-off sensing means is to be detected, as the speed of the movement of the piston member becomes lower, the accuracy of detection by the sensing means becomes increasingly more affected by the sensitivity of the sensing means, as well as the signal producing capability of the sensing portion, and becomes increasingly more prone to errors. For instance, when the arrival of a piston member at a prescribed position such as a stroke end of the piston member or a point adjacent thereto is to be detected by using a fixedly secured reed switch, the lower the speed of the movement of the piston member is, or, in other words, the more gradual the movement of the piston member is, the stronger the fluctuation in the position of the piston member at which the reed switch is activated, the position of the piston member at which the reed switch is activated being more strongly affected by the sensitivity of the sensing means and the signal producing capability of the sensing portion. This means the lack of reliability in the detection of the amount of the remaining content.